1. Field of the Invention
This invention relates to a method and instrumentation for implanting an acetabular cup which is to be held in place by polymethyl-methacrylate (PMMA) bone cement.
2. Description of the Prior Art
It is known, for example, to provide a flexible flange around the rim of an acetabular cup to pressurize the cement, but this only occurs as the cup approaches its final position in the acetabulum. The flexible flange touches the acetabular margin tending to occlude cement flow.
Another device for pressurizing cement is known as the Exeter Acetabular Pressurizer. In this arrangement a water filled seal mounted on the end of a handle is used after the acetabulum has been filled with cement, but before the cup is introduced, the seal covers the acetabulum and the surgeon pushes on the handle thereby forcing the doughy cement in the holes, ridges and trabeculae of the acetabulum. When the instrument is removed and the pressure is released, blood flow can squeeze between the cement and the bone.
It is also known to provide cup positioning instruments with long pointers (vertical, horizontal and at right angles to the patient's center line of body symmetry) to enable the surgeon to control cup orientation reasonably well provided the pelvis does not move. Linear positioning of the cup is determined by eye in a medio-lateral and anterio-posterior sense. If the natural acetabulum is well formed, an eyeball judgment can be acceptable, but many acetabulae are deformed or eroded, in which case eyeball judgment can be confused.
A further method is to use what are usually referred to as acetabular spacers. These can be lugs or PMMA spacers (see for example, U.S. Pat. Nos. 4,883,490 and 4,955,325) fixed to the outside of the acetabular cup or can be press studs pushed into the bone base of the natural acetabulum. These prevent the cup from being pushed too deeply into the acetabular socket and can control medio-lateral and anterio-posterior positioning. However, they force some degree of centralization of the cup. If the surgeon chooses to place the cup eccentrically to make use of a particular area of solid bone support, acetabular spacers may not permit such a placement.